1. Field of the Invention
The present invention relates to a method for producing .alpha.-L-aspartyl-L-phenylalanine methyl ester hydrochloride (abbreviated as .alpha.-L-L-APM.HCl hereafter) by crystallizing .alpha.-L-aspartyl-L-phenylalanine methyl ester hydrochloride from an aqueous solution of L-aspartyl-L-phenylalanine and/or the methyl ester derivative thereof (including an N-protected type) and L-aspartyl-D-phenylalanine and/or the methyl ester derivative thereof (including an N-protected type) which contains hydrochloric acid and methanol, with or without stirring.
2. Discussion of the Background
.alpha.-L-L-APM is a low calorie sweetener of high quality, while .alpha.-L-aspartyl-D-phenylalanine methyl ester hydrochloride (abbreviated as .alpha.-L-D-APM.HCl hereafter) is tasteless and, thus, has no use as a sweetener. A variety of methods are known for the production of .alpha.-L-L-APM.
For example, hitherto known methods include: (a) the condensation of a N-protected-L-aspartic anhydride and the methyl ester of L-phenylalanine, followed by elimination of the N-protecting group according to a conventional method (U.S. Pat. No. 3,786,039); (b) the condensation of a N-protected-L-aspartic anhydride and L-phenylalanine, elimination of the N-protecting group according to a conventional method, followed by crystallization of .alpha.-L-L-APM.HCl in an aqueous solution of methanol (Japanese Unexamined Patent Publication SHO 49-41425); (c) the direct condensation of the methyl ester of L-phenylalanine with a strong acid addition salt of L-aspartic anhydride (Japanese Unexamined Patent Publication SHO 49-14217); (d) the condensation of a N-protected L-aspartic acid and the methyl ester of L-phenylalanine in the presence of an enzyme, followed by conventional elimination of the N-protecting group (Japanese Unexamined Patent Publication SHO 55-135595), etc.
The starting materials for these production methods are L-aspartic acid (abbreviated as L-Asp hereafter) and L-phenylalanine (abbreviated as L-Phe hereafter). L-Asp is easily produced by an enzymatic reaction of the substrates fumaric acid and ammonia and is available at a low cost. On the other hand, regarding L-Phe, although DL-phenylalanine (abbreviated as DL-Phe hereafter) may be synthesized cheaply, the cost for the requisite optical resolution is high, and the production of L-Phe by fermentation requires expensive purification for the removal of the by-products, resulting in a very costly method for producing .alpha.-L-L-APM.HCl, due to the use of the starting material, L-Phe. If DL-Phe were suitable for use in a method for producing of .alpha.-L-L-APM, then the method would be expected to be an advantageous one from an industrial viewpoint.
In this regard, production methods are already known which use L-Asp and DL-Phe as the starting materials. For example, a report has been made of a method where a N-protected-L-Asp and DL-Phe are condensed conventionally to produce N-Protected-.alpha.-L-aspartyl-L-phenylalanine (abbreviated as N-protected-.alpha.-L-L-AP hereafter) and N-protected-.alpha.-L-aspartyl-D-phenylalanine (abbreviated as N-protected-.alpha.-L-D-AP hereafter), after which the N-protected-.alpha.-L-L-AP is separated by crystallization (Japanese Unexamined Patent Publication SHO 63-445945), then the N-protecting group is eliminated in a conventional manner, thereby leading to .alpha.-L-L-APM. This production method, however, has a fatal drawback in that reliance on the difference in solubility between N-protected-.alpha.-L-L-AP and N-protected-.alpha.-L-D-AP prevents the obtaining of N-protected-.alpha.-L-L-AP in high yields.
Needless to say, the above drawback of the N-protected-AP cannot be overcome even if the N-protecting group is first eliminated from the N-protected-.alpha.-L-L-AP and N-protected-.alpha.-L-D-AP in the solution to yield a solution containing .alpha.-L-L-AP and .alpha.-L-D-AP, after which .alpha.-L-L-AP is subjected to conventional dissolution followed by cooling to crystallization. In other words, the isomers must be separated before removal of the N-protecting group, thus presenting industrial problems such as low yields and complicated operations for crystallization and separation.
Thus, there remains a need for a method for producing .alpha.-L-L-APM whereby a decrease in the yield of the desired .alpha.-L-L-APM due to the formation or presence of the byproduct L-D-AP (or its methyl ester derivative) is prevented and .alpha.-L-L-APM.HCl is crystallized directly from a reaction mixture which contains N-protected- or unprotected-L-D-AP (or its methyl ester derivative), selectively and in high yield, in the case where L-Phe containing DL-Phe and D-phenylalanine (abbreviated as D-Phe hereafter) is used as the starting material for production of the .alpha.-L-L-APM.